In certain water treatment applications, there is a requirement for the production of extremely pure water, including the removal of both dissolved and suspended or colloidal materials. One area where such a requirement is particularly important is in the steam generation of electrical power, in both fossil fuel and nuclear power plants. In both types of installations, it is common practice to include a filter bed in the recycle stream for the steam turbines.
A significant breakthrough in the purification of such liquid streams is described and claimed in U.S. Pat. Nos. 3,250,702 and 3,250,703, both of which are assigned to the assignee of this application. The invention described in these patents is based on the discovery that when finely divided anion and cation exchange resin particles are mixed in aqueous suspension, a volume increase is noted. This volume increase is the result of an agglomeration or "clumping" between the anion and cation exchange resin particles. Such resin particles, when used to form a filter bed, produce significantly reduced pressure drops across the bed, together with longer run lengths and improved efficiency of dissolved and undissolved solids removal.
As used herein, the term "bed" refers to a layer, such as a precoat layer, which has been deposited on a filter screen, a wound, annular filter cartridge, a film, a deep or shallow bed, or the like, such a bed may advantageously be deposited on a tubular filter cartridge such as those described in U.S. Pat. Nos. 3,279,608 and 3,779,386, which are assigned to the assignee of this application. Filter presses, vertical and horizontal leaf filters as well as sand and multilayer filters may also be used.
Although ion exchange resin mixtures form a highly efficient filtration system, the ion exchange resins are expensive, and, in some instances, ion exchange capacity is unnecessary. That is, it is sometimes desired to employ a filter having a reduced pressure drop and increased efficiency primarily for the removal of finely divided suspended particles ("crud") from liquids. There are also many instances where it is desirable to employ a non-ion exchange material overlay over the above-described mixed ion exchange resins. Such an overlay is particularly desirable when the liquid being filtered contains suspended particles which interfere with the ion exchange resin. Another advantage to the use of an overlay on precoated filters is that backwashing of the filter to remove the precoat is generally easier than it is when the overlay material is used alone.
A method for removing impurities from a liquid by passing the liquid through a filter bed which comprises a mixture of oppositely charged particles of filter aid material is described and claimed in U.S. Pat. No. 4,177,142, which is assigned to the assignee of this application. The filter aid materials are disclosed as including diatomaceous earth, cellulose fibers, charcoal, expanded perlite, asbestos fibers, ion exchange resins, and inorganic ion exchangers. The filter bed comprises a mixture of oppositely charged particles of filter aid material. The filter aid particles normally have a surface charge in aqueous suspension, and a portion of the particles is treated with a chemical compound to produce a surface charge which is opposite to the normal surface charge. A mixture of oppositely charged particles (normal and reversed) is therefore produced, and the "clumping" phenomenon is achieved.
A method for removing impurities from a liquid by passing the liquid through a filter bed which comprises treated filter aid material which is mixed with finely divided ion exchange resin particles in the size range of 60 to 400 mesh is disclosed and claimed in U.S. Pat. No. 4,190,532, which is assigned to the assignee of this application. The mixture of treated filter aid material and ion exchange resin particles produces a clumping phenomenon. The filter aid materials are disclosed as including cellulose fibers, diatomaceous earth, charcoal, expanded perlite, asbestos fibers and polyacrylonitrile fibers. The ion exchange resin particles are disclosed as including cation exchange resins, anion exchange resins and a mixture of both. The filter aid materials is treated with a chemical compound to produce the required surface charge.
A method for removing impurities from a liquid by passing the liquid through a filter bed which comprises a mixture of treated fibrous filter aid material and an active particulate material is disclosed in U.S. Pat. No. 4,238,334, which is assigned to the assignee of this application. The "fibrous filter aid materials" are disclosed as including cellulose fibers, polyacylanitrile fibers, Teflon fibers, nylon fibers, rayon fibers, polypropylene fibers, and polyvinyl chloride fibers. The "active particulate materials" are disclosed as including activated carbon, adsorptive clays such as bentonite, molecular sieves such as zeolites, zirconium oxides, zirconium phosphate, iron sulfide, diatomaceous earth, synthetic adsorbants and activated alumina. The filter aid material is treated with a chemical compound to produce the required surface charge.
In U.S. Pat. No. 3,880,754 a method for removing impurities from a liquid is disclosed which comprises passing the liquid through a bed of a cation and an anion exchange resin mixed with a filter aid material comprising ground fibers of polyacrylanitrile, a polyamide or a mixture thereof. There is no disclosure of treatment of the filter aid material to alter its surface charge.
There is no disclosure in the prior art of the use of polyester fibers as a filter aid material in the formulation of filter beds used in the removal of impurities from liquids passing therethrough. Polyester fibers are non-biodegradable and therefore filter beds formulated therefrom are more easily disposed of. Polyester fibers are less compressible than the cellulosic fibers, most commonly used as filter aid materials in filter bed formulations, and hence the pressure drop of polyester fiber containing filter beds is less than the pressure drop of cellulosic fiber containing filter beds. Polyester fibers are more chemically stable than the cellulosic fibers, and hence may be used at extremes of pH and temperature which would degrade the cellulosic fibers.
However, despite the above-mentioned advantages of the use of polyester fibers as a filter aid material in formulating filter beds, it has not heretofore been practical to substitute such fibers for the commonly used cellulosic fibers. It has been found that polyester fibers are too hydrophobic to be of practical use in filter bed formulations, particularly when such formulations are precoated onto a filter screen or tubular filter cartridge. Prior attempts to use polyester fibers as a filter aid material in filter bed formulations have encountered problems with unacceptable high levels of floating fibers and poor precoatability and backwashability of the filter bed formulations. Until these problems were solved, polyester fibers could not be successfully used as a filter aid material in filter bed formulations.